SPECTRAL SIGNATURES OF WATER BODIES WITH DIFFERENT TURBIDITIES 
by 
K.R.Rao, B.L.Deekshatulu, K.Krishnam Raju 
National Remote Sensing Agency (NRSA), Secunderabad, India 
Abstract: This paper presents analysis of the data pertaining to 
different water bodies having relative turbidities varying from 
3 to 25 and the corresponding spectral reflectance values. Secchi 
depth measurements and bottom reflectance effect measurements were 
taken. The depth of extinction was determined for waters of different 
turbidity values. The data was collected on five water bodies 
around Mandya town which is about 95 kms. south of Bangalore on 
Bangalore-Mysore highway. 
Correlation of ground data with that of the M?S data collected at 
the same time has been attempted. From the data (Fig.1, 2) it is 
established that the spectral reflectance increases with increasing 
turbidity. This phenomenon is more prominent in bands 4 and 5 
(i.e. 0.5-0.6um; 0.6-0.7um). The spectral reflectance increases 
much faster in the lower turbidity values. It is also established 
that the absorption of radiation in water is of exponential 
nature (Fig.16-19). 
1. Introduction: 
1.1 There are a number of papers published in recent years re- 
lating to water quality and corresponding spectral signatures 
measured from the ground as well as from aircraft and spacecraft. 
Ritchie Etal (4) studied the reflected and incident solar radiation 
on six northern Mississippi reservoirs and showed that 0.7 to 
0.8um is the best band for suspended solid concentration studies. 
They have also established the fact that the sun angle has a 
definite effect on the reflectance relationships. Mc Cluney etal 
(3) have discussed the approaches for development of techniques 
for the analysis of remote multispectral imagery of natural 
resources of water. A set of maximum surface truth measurement 
parameters and techniques are suggested in this paper. Bartolucci, 
L.A. etal (5) measuring spectral reflectances of a highly turbid 
water body and a low turbidity clear water body, established the 
increases of spectral reflectance with turbidity. Bottom reflec- 
tance effect has also been measured. 
1.2. Turbidity: Turbidity is caused by the presence of colloidal 
particles in water. Colloidal particles are larger than true ions 
and molecules dissolved in liquids and smaller than settleable 
solids. Colloidal suspensions have some unique properties. They 
scatter light and the amount of light scattered is related to 
the degree of turbidity and particle surface area. Sources of 
turbidity in water are clay, silt soil, sewage bacteria, indus- 
trial wastes, etc. The concentration of turbidity is measured 
by the degree of light scattered by the sample. The turbidity 
values given here are not absolute values but are relative.